This invention pertains to systems for pneumatically conveying granular materials and, more specifically, to dense phase pneumatic conveying systems. More particularly, the invention is directed toward providing means for controlling and reducing the rush of the conveying gas at the end of the conveying cycle.
Pneumatic conveyors are used to transfer or carry a wide variety of granular materials such as sand, salt, flour, cement, etc. Basically, there are two general types of pneumatic conveying systems, dense phase and dilute phase. The basic difference is that the latter usually operates under a relatively low pressure but at a high velocity whereas the former operates at a higher pressure but a lower velocity. The instant invention is directed principally for use in the dense phase system. The dense phase system is best suited for handling heavy abrasive products and non-abrasive products where degradation cannot be tolerated. Because of the lower velocities at which the material is transported, the wear on the system components is minimized. In the dense phase system a positive pressure using a relatively small amount of gas is used to move a large amount of material. The conveying line is generally flooded with the granular material to be conveyed and then, by adding just the right portion of pressurized gas, usually air, to overcome frictional losses in the conveying line a dense flow is produced. The air does not necessarily carry the material in an air stream but has more of a pushing effect, much like extruding. In the dense phase flow the conveyed material is carried at a much slower velocity than what the air velocity would otherwise be or what the velocity of material flow would be in a dilute phase system.
In general the pressurized conveying air is applied at the source of the material, that is, in the pressure vessel where the material is temporarily held, sometimes referred to as a transporter, and the receiving vessel or container is usually at atmospheric pressure. Additional booster air may be applied along the conveying line to assist in conveying the granular material. When the source vessel and the conveying line are filled with the material, the pressurized air applied at the source vessel sees a vary high resistance and so the air flows at a relatively slow rate. However, as the conveyor approaches the end of the conveying cycle, that is, after most of the material has been moved to the receiver, then the resistance to the pressurized air decreases so the air now passes through the conveying line to the receiver at a greater velocity. Finally, right at the end of the conveying cycle the air rushes with a surge through the conveying line and arrives at the receiver with an impact. In general, at the receiver dust particles must be removed from the air before it is allowed to pass out to the atmosphere. Therefore, the dust collector or filter at the receiver has to be constructed to take the force of the impact due to the surge of air that arrives at the end of the conveying cycle and must also be capable of filtering out this rush of air.